The present invention relates to a method and apparatus for measuring gas emission flux from a surface and, in particular, to a method and apparatus for measuring the emission of methane and other organic gases from municipal solid waste landfills.
It has been common practice to bury solid waste products either in municipal solid waste (MSW) landfills or in various government or private hazardous waste landfills. Awareness of the leakage and production of gas emission from such landfills led to the development, under grants from the Environmental Protection Agency (EPA), of a flux chamber for measuring emission rates of various organic gases and vapors from contaminated soil. Since the EPA identifies the flux chamber as a recommended method for estimating baseline emissions from Superfund sites, the flux chamber has become a generally accepted standard. Other methods are known, such as drilling sample wells (U.S. Pat. No. 4,026,355), excavating the surface (U.S. Pat. No. 4,487,054), and measuring soil gas pressures and soil permeabilities (U.S. Pat. No. 5,063,519). A device similar to the flux chamber is shown in U.S. Pat. No. 4,880,973 in an application for sampling radon gas.
The basic flux chamber is described in the Journal of the Air and Waste Management Assoc., Vol. 42, Dec. 1992, pages 1583 et seq. The flux chamber comprises a closed-top chamber with no bottom that is inserted part way into the soil to enclose a defined landfill surface area. A controlled and measured flow of clean, dry sweep air from a compressed air cylinder is introduced into the flux chamber at a rate significantly exceeding the organic gas release rate from the surface. This sweep air mixes with the gaseous emissions from the enclosed landfill surface and transports these gases through an exit port. The total concentration of the methane and other organic gases is measured in the exit air stream directly using a portable gas analyzer. The gas emission rate from the surface is calculated from the measured concentration, the known flow rate of sweep air and the surface area enclosed by the chamber. The emission rate is commonly referred to as a flux rate and is calculated by multiplying the flow rate of sweep air by the measured concentration and dividing the result by the surface area under the chamber. Alternatively, the gas can be captured in a sample container and later analyzed in a laboratory to calculate the emission rate of each species. In cases where the gases in the flux chamber exit stream have been diluted below detection limits, a sample of whole landfill gas can be withdrawn from under the surface for later analysis.
Use of the flux chamber has been restricted by a lack of quality assurance testing, physical difficulties in using the technique, soil disturbances created when inserting the device, extensive time required for testing over large areas, and a tendency to underestimate fluxes due to excessive chamber pressure.
Another significant detriment of the flux chamber method is the need to transport a relatively large chamber to various test sites along with conventional cylinders of clean sweep air and other equipment.
An alternative to flux chamber sampling is a method developed for the California Air Resources Board, called the Integrated Surface Sample (ISS) method. In this method, a person walks a prescribed path holding a hooded sampling wand connected to an instrument that draws a continuous sample of air from near the surface. It is very important to the success of this method that the hooded intake be held exactly a constant distance, typically four inches, from the surface as the person walks.
The ISS method suffers from several operational difficulties, primarily keeping a constant distance. Also, the measured concentrations are highly variable depending on the operator, the shape of the surface and ambient conditions such as the wind. Furthermore, the ISS measurement cannot easily be translated into an emission rate from the surface. Typically, it is used simply to identify compounds being emitted. While the ISS method is faster and easier than the flux chamber method, it is much less accurate and reproducible.